Movement and Behaviour of Cercariae

Examination of liberated cercariae in an undisturbed finger bowl shows that they are located in greatest numbers opposite the exhalent chamber. The cercariae are generally in a resting position with the furcae contracted, encircling the body, and almost meeting in front of the anterior extremities (Text-fig. 6, B).

The furcae rapidly expand and tend to coil on stimulation by light (Text-fig. 6, F). If the water is then agitated any one of the positions illustrated in Text-figs. 6, A, C or E is assumed with the body suspended vertically and the furcae streaming above. The angle between the furcae is acute when the cercaria is nearer the bottom of the bowl, but obtuse nearer the surface. If strong currents are set up in the water the furcae bend with the current (Text-fig. 6, A). The sudden flexure and coiling of one furca of a suspended cercaria results in horizontal movement in calm water (Text-fig. 6, D). A cercaria may remain suspended for at least one hour in calm water, but after this time slowly sinks to the bottom of the bowl and coils the furcae which are eventually fully contracted to revert to the resting position (Text-fig. 6, B).

Light tapping of a finger bowl containing cercariae with the furcae expanded causes immediate and complete contraction of the furcae. The furcae normally revert to their expanded state when the tapping ceases. A similar reaction to mechanical shock has been described by Woodhead (1930) for the cercaria Bucephalus elegans. By comparison with the present species, however, B. elegans requires about three minutes to recover from the shock. Dawes (1946) does not describe the reaction of B. polymorphus to mechanical shock but he does describe the position of the furcae in a resting cercaria which is identical with that of the present species. Both Woodhead and Dawes stated that B. elegans and B. polymorphus respectively, are able to swim by alternate contractions and expansions of the furcae, but this was not observed in the present species.

Hopkins (1954) and Holliman (1961) stated that true swimming movements of Cercaria cuculus and C. apalachiensis do not occur and the cercariae in each case rely on turbulence in the water to remain suspended. With mechanical shock, the furcae of the latter species violently contract to about twice body length and are temporarily orientated at about 60° to the body in a rod-like manner. The resultant appearance of this cercaria can be contrasted with that described above page 21

Text-fig. 6.—Bucephalus longicornutus. Behaviour of the cercaria: Fig. A, suspended cercaria (semi-diagrammatic) in a strong current; Fig. B, resting position or position induced by light mechanical shock; Fig. C, suspended cercaria some distance below the surface of the water; Fig. D, suspended cercaria undergoing lateral movement by flexure of one of the furcae; Fig. E, suspended cercaria near the surface of the water; Fig. F, relaxed position assumed with the furcae expanded and partly coiled. For abbreviations see p. 9.

page 22 for the present species, as well as B. polymorphus and B. elegans. It is suggested that this contrast may reflect a generic distinction between C. apalachiensis and the three Bucephalus cercariae. In fact, behaviour of the furcae may reflect the generic identity of a given cercaria.

Body movements, independent of furcae movements, take place. From a contracted state a peristaltic wave commences at the anterior extremity of the body. When this has reached about one-third body length from the anterior extremity, a similar wave commences at the posterior extremity. When these two waves meet at approximately mouth level, the body of the cercaria is fully expanded. This is followed by overall contraction of the body to revert to the initial position.